1. Field of the Invention
The invention relates to a fluxless joining process utilizing ion beam milling and reflow techniques.
2. Prior Art
In the preparation of present day large scale integration technology, both for semiconductor and Josephson type devices, it is necessary to interconnect circuit chips to a substrate. Normally, solder joints are used in order to make these interconnections. The present day procedure entails the placement of solder pads on the chip which are to be joined to metal or solder pads which have been established in the substrate. The solder pads are treated with a wet chemical flux in order to dissolve surface oxide which forms thereon. The flux treatment serves to permit solder flow upon heating.
This prior art method has a severe drawback in that the flux is most difficult to remove and remanents thereof remain as a corrosive contaminant. Additionally, to join the flux treated solder pads it is necessary to heat the entire chip assembly.
More recently, it has been recognized that surface oxide can be removed from solder pads by means other than the use of chemical fluxes. For example, in the publication to R. J. Herdizik et al., IBM Technical Disclosure Bulletin, Vol. 23, #11, April 1981, it is recognized that ion milling can be used to effect solder milling i.e., removal of the oxide film. The so treated solder is then heated on a heating stage to effect solder reflow.
In U.S. Pat. No. 3,294,951 to K. O. Olson, there is described a method for micro-soldering using a finely focused electron beam. This method cannot be used for large scale integration as contemplated by the present invention. This method cannot separately remove oxides and cause solder reflow, as well as it cannot be used to cover large areas simultaneously. Additionally, because electron beams are highly energized, they can be damaging to sensitive components.